It is well known that cinematographic film suffers degradation from the presence of dust and dirt on its surface. This can occur when the film is transferred into a video signal in a telecine, for broadcast transmission or recording on videotape. The dust or dirt then leads to black or dark marks in the image. Alternatively, degradation can occur during film copying, particularly from negative to positive, in which case white or light marks result. Film stock can also suffer similar types of defect for other reasons; for example aging of the film emulsion can lead to `sparkle` in monochrome (black-and-white) film, as the silver falls off the film substrate.
It is desirable to be able to conceal these defects so as to improve the subjective image quality of the pictures for the viewer. One prior method is described in BBC Research Department Report RD 1985/4 "Electronic Detection and Concealment of Film Dirt", R. Storey, published by The British Broadcasting Corporation, Research Department, Kingswood Warren, Tadworth, Surrey, KT20 6NP, United Kingdom, and in United Kingdom Patent Application GB-A-2 139 039 BORER, the disclosure of which is incorporated by reference. In that method dirt is detected by comparison of the video signal derived from corresponding areas in three successive frames of a cinematographic film to determine whether the signal from the middle one of the three frames has a value which is substantially different from the values for the preceding and succeeding frames. If so, it is assumed that dirt is present on that area of the middle frame. The threshold amount by which the value of the signal in the middle frame differs from that to either side can be adaptively adjusted dependent upon signal content. Motion detection is used to detect the presence of movement in the scene content and to increase the threshold in the presence of movement.
This system is quite effective but does still have is certain limitations. In particular:
(I) it fails to conceal dirt in areas of high motion content, and PA1 (ii) it can introduce motion artifacts.
Another blemish concealment method is described in United Kingdom Patent Application GB-A-2 202 706. In this proposal a median selector is used. This is a circuit which has three inputs and selects as its output that one of the inputs which has the middle (median) value of the three input values. The median selector receives signals from three successive frames. By selecting the median value, extreme values caused by blemishes such as dirt are eliminated. Motion compensation is used. The signals from the `preceding` and `succeeding` frames are spatially displaced in dependence upon the output of a motion measurement circuit, so as to attempt to eliminate the effect of movement where this occurs.
This method introduces approximately 3 dB of noise reduction in the output signal. This is sometimes undesirable.